Polymer metathesis is a well documented operation, as disclosed, for example, in US 2003/0027958 A1, US 2003/0088035 A1 and US 2004/0132891 A1.
More specifically, certain ruthenium-containing catalysts are known to be particularly suitable for the selective metathesis of nitrile rubber, i.e. the cleavage of the carbon-carbon double bonds without concomitant reduction of the carbon-nitrogen triple bonds present in the nitrile rubber.
For example, US 2003/0088035 A1 teaches the use of bis(tricyclohexylphosphine)benzylidene ruthenium dichloride in such a process resulting in a nitrile rubber with a reduced molecular weight. Similarly, US 2004/0132891 A1 teaches the use of 1,2-bis-((2,4,6-trimethylphenyl)-2-imidazolidinylidene)(tricyclohexylphosphine)-ruthenium(phenylmethylene)dichloride for the metathesis of nitrile rubber, although in the absence of a co-olefin. In both of these processes the nitrile rubber is first dissolved in a suitable solvent to provide a viscous rubber solution. If desired a co-olefin is added to the reaction solution. The catalyst is then dissolved in the rubber solution. Following the metathesis of the nitrile rubber the rubber solution can be optionally hydrogenated to hydrogenated nitrile rubber (“HNBR”) 30 using known hydrogenation techniques, as disclosed, for example, in U.S. Pat. No. 4,464,515, and GB-A-1,558,491.
While the polymer metathesis itself is a well documented process, this does not apply to the post-metathesis separation of the metathesis catalyst from the polymer.
Even with regard to hydrogenation of unsaturated nitrite rubbers only a limited number of publications are dealing with or even mentioning the separation of the hydrogenation catalyst from the reaction mixture and/or the hydrogenated nitrile rubber.
U.S. Pat. No. 4,464,515 teaches the use of hydrido rhodium tetrakis (triphenylphosphine) catalyst, i.e. HRh(PPh3)4, in a process to selectively hydrogenate unsaturated nitrite rubber. The unsaturated nitrite rubber is first dissolved in a suitable solvent to provide a viscous rubber solution, The catalyst is then dissolved in the rubber solution. The hydrogenation process is said to be homogeneous because the substrate and catalyst are contained in the same phase. The HNBR obtained is precipitated and simply washed with iso-propanol. There is no further disclosure about removing the hydrogenation catalyst.
GB-A-1,558,491 teaches the use of chloro rhodium tris(triphenylphosphine) (RhCl(PPh3)3) as catalyst in a process to hydrogenate unsaturated nitrite rubber. The hydrogenation product is separated off from the reaction solution by treatment with steam or by pouring into methanol and is subsequently dried at elevated temperature and reduced pressure. Once more no teaching is given how the hydrogenation catalyst might be removed.
U.S. Pat. No. 6,376,690 discloses a process for removing metal complexes from a reaction mixture and it is said that such process is especially amenable for the post-reaction separation of ruthenium and osmium metathesis catalysts from the desired products. Said separation process in which a second immiscible solution containing a solubility-enhancing compound (preferably a phosphine or derivative thereof) is added to the original reaction mixture. The metal catalyst once reacted with the solubility-enhancing compound migrates out of the reaction mixture into the second solution. This solution is then removed from the reaction solution.
While U.S. Pat. No. 6,376,690 teaches for the removal of metals like Cu, Mg, Ru, and Os, it involves the addition of additives which, if not fully removed, can interfere in any subsequent reaction step, like e.g. with the hydrogenation catalyst used in a subsequent hydrogenation reaction. Secondly, the separation of two immiscible solutions while relatively easy on small scale is quite a complex process on a commercial scale of grand scale.
WO-A-2006/047105 discloses the separation of a metathesis catalyst from a reaction mixture through contact of the reaction mixture with a nanofiltration membrane. The reaction mixture contains not only the metathesis catalyst, but in addition one or more unconverted reactant olefins, optionally a solvent and one or more olefin products. As nanofiltration membranes a polyimide membrane is preferably used so as to recover a permeate containing a substantial portion of the olefin reaction products, the unconverted reactant olefins, and optional solvent, and a retentate containing the metathesis catalyst, and optionally, metathesis catalyst degradation products. The process of WO-A-2006/047105 is considered to be applicable to homogeneous metathesis catalysts on the basis of ruthenium, molybdenum, tungsten, rhenium, or a mixture thereof, preferably on the basis of ruthenium. WO-A-2006/047105 does not comment on the possibility of utilizing such a membrane technology for the removal of a rhodium species also. Therefore in the situation were said nitrile rubber is hydrogenated in the next step two separate metal catalyst recovery processes would probably be needed resulting in considerable cost increases and negative capacity results.
Organic Letters, 2001, Vol. 3, No. 9, pages 1411-1413 describes a method for removing undesired highly colored ruthenium byproducts generated during olefin metathesis reactions with Grubbs catalysts. The crude reaction products like diethyl diallylmalonate obtained by ring closing metathesis are treated with triphenylphosphine oxide or dimethyl sulfoxide, followed by filtration through silica gel. This allows to remove the colored ruthenium-based byproducts which is important as an incomplete removal is known to cause complications such as double bond isomerization during distillation or decomposition of the reaction products. However, as with U.S. Pat. No. 6,376,690, the use and introduction of additives such as dimethyl sulfoxide could—if not completely removed after its use—be detrimental if applied to solutions of nitrile rubber which shall then be subjected to a subsequent hydrogenation. A transfer of such process to nitrile rubber solutions is therefore not a viable alternative. Additionally the necessary silica gel filtration process in terms of a commercial process would result in extensive costs.
In Tetrahedron Letters 40 (1999), 4137-4140 it is disclosed to add a water-soluble tris(hydroxymethyl)phosphine to a reaction mixture which contains diethyldiallylmalonate obtained by ring closing metathesis in the presence of the Ru-catalyst Grubbs I. It is observed that when the crude reaction mixture is added to a solution of tris(hydroxymethyl)phosphine and triethylamine in methylene chloride, the solution turned from a black/brown color to pale yellow within five minutes, this indicating that tris(hydroxymethyl)phosphine was coordinating to the ruthenium. Upon the addition of water, the yellow color moved into the aqueous phase leaving the methylene chloride phase colorless. NMR studies showed that all of the desired product remained in the methylene chloride phase and all of the phosphine moved to the aqueous phase. In an alternative embodiment the diethyldiallylmalonate solution containing the ruthenium catalyst byproducts was stirred with a solution of tris(hydroxymethyl)phosphine, and triethylamine in methylenechloride in the simultaneous presence of excess silica gel. As the tris(hydroxymethyl)phosphine is know to graft onto silica gel this gave the best results.
The recovery of rhodium complexes from non-viscous chemical process streams using ion-exchange resins is also known. For example, DE-OS-1 954 315 describes the separation of rhodium carbonyl catalysts from (low molecular weight) oxo reaction mixtures by treating the raw oxo reaction mixtures with a basic ion exchanger in the presence of CO and hydrogen.
Chemical Abstracts 85: 5888k (1976) teaches the use of a thiol-functionalized resin to recover Group VIII noble metal complexes which have been used as catalysts in hydrogenation, hydroformylation and hydrocarboxylation. Organic solutions containing said catalyst residues are treated with ion-exchange resins.
Chemical Abstracts 87: 26590p (1977) describes a two-stage process in which (i) an aqueous, noble-metal containing solution is prepared by extracting the noble metal from a waste ceramic catalyst carrier and (ii) the noble metal is then adsorbed on an ion-exchange resin.
Eventually, Chemical Abstracts 95: 10502r (1981) relates to the simultaneous recovery of platinum and rhodium by extracting the metals from spent catalysts using HCl and HNO3, followed by the subsequent use of an ion-exchange column to adsorb the metals.
U.S. Pat. No. 4,985,540 discloses a process for removing rhodium-containing catalyst residues from hydrogenated nitrile rubber by contacting a functionalized ion exchange resin with a hydrocarbon phase, which contains the hydrogenated nitrile rubber, the rhodium-containing catalyst residues and a hydrocarbon solvent. It is said that such process is capable of removing rhodium from viscous solutions containing less than 10 ppm rhodium (weight rhodium/weight solution basis). The ion exchange resins used preferably have a relatively large average particle diameter between 0.2 and 2.5 mm.
In U.S. Pat. No. 6,646,059 B2 it is disclosed to remove iron- and rhodium-containing residues from a solution of hydrogenated nitrile rubber which has been obtained by hydrogenating a nitrile rubber in the presence of a rhodium-based catalyst. Iron residues may occur in the solution of the optionally hydrogenated nitrile rubber due to minimum corrosion occuring in the reaction vessels or pipes, especially if the preparation of hydrogenated nitrile rubber is performed using a catalyst containing chloride, like e.g. Wilkinson's catalyst (Cl—Rh[P(C6H5)3]3), and HCl is therefore formed as a bi-product during hydrogenation. Alternatively iron residues may occur due to the fact that iron-containing compounds might have been used as activators in the polymerisation of the nitrile rubber. The process of U.S. Pat. No. 6,646,059 B2 utilizes a specific monodispersed macroporous cross-linked styrene-divinylbenzene copolymer resin having thiourea functional groups. The fact that the ion-exchange resin is monodispersed is important for the successful performance of the process.
In view of the fact that the variety of catalysts which may be used in the preparation of optionally hydrogenated nitrile rubber has steadily increased during the last years, there remains a need for finding new methods for removing metal-containing catalyst residues from optionally hydrogenated nitrile rubber, particularly with respect to viscous solutions of optionally hydrogenated nitrile rubber.